Before I start I just want to say that you guys rock and the info you give is very appreciated.

So here's the set up. I will be getting a Brahma 15 soon from a friend. I plan to use this sub as the main sub in my home theater. So I need to build a box. I have plenty of room to use here, so size isn't too much of an issue. I was thinking 5 cubic feet with a slot port at 20Hz (Thats what I've read to do I guess). I am not sure how to make a slot... I've only made round ports before, but I want a slot vent. Also, I have never braced a sub. So if someone could give me info on how to brace a sub in a ported box, and how to make a slot vent that would be great. Do you use insulation in a ported box? Is that necessary? What dimensions would make a box 5 cubic feet? I am totally a beginner on trying to use the correct dimensions of the driver for the box I make. In the past I've just made boxes by guessing. But with this new beast, I want to make it the best as possible. Thanks for your time to read this, and any additional information you'd like to give me would be great too. Thanks.

A 5 cu.ft ported box with a slot port at 20 Hz will be around 24"x24"x24".

You don't need insulation in a ported box.

It will take at least a full sheet of 3/4" MDF or plywood, 4' x 8'.

To brace a box, you need to put full length panels inside the box with alot of 4" holes into them. You could look on Adire Audio, there's already a plan for a braced 5 cu.ft box, the Shiva EBS 142.5 liters, you'll need to modify it for a slot port since it's designed for a flared 4" port.

A slot or round port is identical in operation.
Use what you are familiar with. On a 15" Brahma, I'd go for a minimum 6" port or dual 4". There is a difference in dynamics and distortion over a small port.

If you ask me.... a bigger box would be nicer. What I typically do when designing a vented box is design for about a 6db/octave rolloff below 30-40hz. Typically this takes a larger box tuned very low, but it usually yeilds fairly flat in room response very low. I'd put that brahma in a 7 cubic ft enclosure tuned between 10hz and 16hz.

One advantage of infrasonic tuning is the fact that port chuffing will rarely occur at any power because the air in the port is typically not fully excited. Another advantage is group delay. If you look at the group delay of the brahma 15 in 5cubic feet tuned to 20hz group delay is above 10ms to 33hz. With a 7 cubic ft enclosure tuned to 10hz group delay is under 10ms above 20hz. Not to say that is plainly audible, but lower tuning yeilds group delay closer to sealed box(at least in the audible part of the frequency spectrum).

The last advantage of tuning infrasonic, is of course, infrasonic response. Its fun to play a 14hz sine and feel the house rattle. Or better yet, when guests are over make them think an earthquake is happening!!!

And of course, there is always multi tuning. Instead of having one port tuned to one frequency use 4 tuned to different frequencys. You CANNOT have four simultanious resonances, but use removable port plugs and you can change tuning easily.